A method and apparatus is described for estimating terminal purity or terminal contamination for a fluid during the withdrawal of the fluid from a subsurface formation. The apparatus and method provide for measuring refractive index of the fluid over a time period, fitting a curve through the refractive index measurements or data values derived therefrom and estimating a terminal refractive index or terminal value for the data values from the fitted to curve to estimate the terminal contamination or purity for the fluid.
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1. A method of estimating a characteristic of brine in a fluid obtained from a formation that has water-based mud invasion, the method comprising:
estimating a refractive index of a connate brine using in part well log data;
withdrawing the fluid from the formation;
measuring a refractive index of the fluid a plurality of times during the withdrawal of the fluid from the formation; and
estimating the characteristic of the brine using the estimated refractive index and the refractive index measured during the withdrawal of the formation fluid.
3. A method of estimating a characteristic of brine in a fluid obtained from a formation that has water-based mud invasion, the method comprising:
estimating a refractive index of a connate brine from well log measurements;
withdrawing the fluid from the formation;
measuring a refractive index of the fluid a plurality of times during the withdrawal of the fluid;
fitting a curve to data values that correspond to the plurality of refractive index measurements; and
estimating the characteristic of the brine from the estimated refractive index and the fitted curve.
8. A method of estimating a characteristic of a fluid, comprising:
withdrawing the fluid from a formation;
measuring a refractive index of the fluid during the withdrawal of the formation fluid to obtain a plurality of refractive index values;
obtaining a plurality of resistivity values corresponding to the plurality of refractive index values;
fitting a curve through the plurality of resistivity values;
estimating a terminal value of the resistivity values from the fitted curve; and
estimating the characteristic of the fluid using a current resistivity value and the estimated terminal value.
24. A computer-readable medium containing a computer program accessible to a processor that executes instructions contained in the computer program, wherein the computer program comprises:
a set of instructions to fit a curve to data corresponding to a plurality of refractive index measurements of a fluid taken during withdrawal of the fluid from a formation;
a set of instructions to estimate a terminal value of the refractive index from the fitted curve; and
a set of instructions to estimate a characteristic of brine in the fluid from the fitted curve and an estimated value of connate brine computed using well log data.
12. An apparatus for estimating a characteristic of brine in a fluid withdrawn from a formation, comprising:
a probe to withdraw the fluid from the formation;
a refractometer that provides refractive index measurements of the fluid during the withdrawal of the fluid from the formation; and
a storage device that has stored therein an estimated value of refractive index of connate brine in the formation that is obtained using well log data; and
a processor that estimates the characteristic of the brine from the estimated value of the refractive index of the connate brine and the refractive index measurements made during the withdrawal of the fluid from the formation.
15. An apparatus for estimating a characteristic of brine obtained from a formation that has water-based mud invasion, comprising:
a probe to withdraw a fluid from the formation;
a refractometer that provides a plurality of refractive index measurements of the fluid during the withdrawal of the fluid from the formation;
a storage device that has stored therein an estimated value of refractive index obtained from well log data; and
a processor that:
fits a curve to data values that correspond to the plurality of refractive index measurements; and
estimates the characteristic of the brine in the withdrawn fluid from the estimated refractive index and the fitted curve.
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This application takes priority from U.S. Provisional Patent Application Ser. No. 60/790,657 filed Apr. 10, 2006.
1. Field of the Disclosure
This disclosure relates to an apparatus and a method for estimating a condition of formation fluid using an index of refraction during withdrawal of such fluid from the formation.
2. Description of the Related Art
Oil and gas wells are drilled while circulating drilling fluid (also referred to as “mud”) in the wellbore. Drilling fluids are typically water-based or oil-based. After drilling the well and before completing the well for the production of hydrocarbons, fluid samples are often withdrawn from the subsurface formation at various wellbore depths to determine the characteristics of the fluid in order to determine the location and fraction or amount of hydrocarbons in the formation fluid and the condition of the reservoirs, etc. In some cases, it also is desirable to obtain samples from the formations or zones that contain mostly formation brine, i.e., water samples.
A majority of the wells are drilled under overbalanced conditions, that is, the wells are drilled wherein the pressure in the wellbore due to the weight of the drilling fluid is greater than the formation pressure. The drilling fluid invades or penetrates to varying depths in the formation, depending upon the physical conditions of the formation being drilled, such as porosity, permeability and other rock properties. This fluid penetration (also referred to as the fluid invasion) contaminates the connate or virgin fluid in the formation. Therefore, before obtaining a fluid sample downhole, a tool is set at the desired depth and the fluid is withdrawn or pumped from the formation into the wellbore until it is determined that the fluid being withdrawn is substantially free of the drilling fluid. Downhole tools referred to as “formation testers” are typically set at the desired depth in the wellbore to pump out the fluid and to withdraw the formation fluid samples. Initially, fluids that are withdrawn from the formation are often highly contaminated with filtrates of the drilling fluid used for drilling the wellbore. To obtain samples that are sufficiently clean (usually <10% contamination) formation fluids are generally pumped from the formation into the wellbore for a period of time, typically 30-90 minutes, before collecting samples in sample chambers for laboratory analysis. Optical sensors are often used to monitor a contamination level in the withdrawn fluid. Optical absorption measurements have been used to estimate how long it might take before relatively clean fluid samples can be taken and to estimate the eventual purity or contamination levels if the fluid is pumped for a relatively long time period. Refractive index measurements have been taken downhole but have not been used to estimate purity or contamination levels for brine. Refractive index measurements can be much less sensitive to the passage of sand particles or other elements present in the formation fluid that may scatter light in the fluid being analyzed than optical absorption spectral measurements.
Therefore, it is desirable to provide an apparatus and method that uses refractive index measurements to estimate one or more characteristics of brine obtained from formations that have water-based invasion, to determine purity or contamination of brine while withdrawing the fluid from a formation, and to estimate at a given time how long it might take before clean up will occur so that a sample may be taken. In certain situations, such as when the invaded zone is too deep or when there is continued penetration of an unwanted fluid from adjacent formations during the pumping of the fluid, it may not be feasible to obtain a clean sample even if pumping were to continue for a relatively long time period. In such cases it is desirable to determine in a relatively short time that it may not be feasible to withdraw samples in a reasonable amount of time from the particular location in the wellbore.
In one aspect, a method for estimating a characteristic of brine present in the fluid obtained from a formation that has water-based mud invasion is disclosed. The method includes: estimating a refractive index of the connate brine from well log measurements; withdrawing the fluid from the formation; measuring a refractive index of the fluid a plurality of times during the withdrawal of the fluid from the formation; estimating the characteristic of the brine by comparing the estimated refractive index of the connate brine with a refractive index measured during the withdrawal of the formation.
In another aspect, a method for estimating a characteristic of brine in a fluid obtained from a formation that has water-based mud invasion is provided wherein the method includes: estimating a refractive index of the connate brine from well log data; withdrawing the fluid from the formation; measuring a refractive index of the fluid a plurality of times during the withdrawal of the fluid; fitting a curve to data values that correspond to the plurality of refractive index measurements; and estimating the characteristic of the brine from the estimated refractive index of the connate brine and the fitted curve.
In another aspect, a method for estimating a characteristic of a fluid obtained from a formation is provided, wherein the method includes: withdrawing the fluid from a formation; measuring a refractive index of the fluid during the withdrawal of the formation fluid to obtain a plurality of refractive index values; obtaining a plurality of resistivity values corresponding to the plurality of refractive index values; fitting a curve through the plurality of resistivity values; estimating a terminal value of the resistivity values from the fitted curve; and estimating the characteristic of the fluid using a current resistivity value and the estimated terminal value.
In another aspect, an apparatus for estimating a characteristic of brine in a fluid withdrawn from a formation is disclosed that includes a probe to withdraw the fluid from the formation; a refractometer that provides refractive index measurements of the fluid during the withdrawal of the fluid from the formation; and a storage device that has stored therein an estimated value of refractive index of connate brine in the formation that is obtained using well log data; and a processor that estimates the characteristic of the brine from the estimated value of the refractive index of the connate brine and refractive index measurements made during the withdrawal of the fluid from the formation.
In another aspect, the apparatus includes a probe to withdraw the fluid from the formation; a refractometer that provides a plurality of refractive index measurements of the fluid during the withdrawal of the fluid from the formation; a storage device that has stored therein an estimated value of refractive index obtained from well log data; and a processor that: fits a curve to data values that correspond to the plurality of refractive index measurements; and estimates the characteristic of the brine in the withdrawn fluid from the estimated refractive index and the fitted curve.
Examples of the more important features of the apparatus and method disclosed herein have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims.
For a detailed understanding of the apparatus and methods described herein, references should be made to the following detailed description of the embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein:
Telemetry for the wireline embodiment includes a downhole two-way communication unit 116 connected to a surface two-way communication unit 118 by one or more conductors 120 within the armored cable 115. The surface communication unit 118 is housed within a surface controller 150 that includes a processor, memory and an output device, collectively designated by numeral 152. A typical cable sheave 122 is used to guide the armored cable 115 into the borehole 101. The tool 103 includes a downhole controller 160 having a processor and memory 162 for controlling formation tests in accordance with methods described herein. The downhole tool 103 includes a plurality of sensors including an optical sensing module 170 and optional sample tanks 128. The optical sensing module 170 is used to measure refractive index of the fluid withdrawn from the formation over time at selected locations and at varying depths within the borehole 101. The tool 103 also included other sensors (generally denoted by number 125), such as pressure sensor, temperature sensor, flow meter, etc.
A downhole controller 160 controls the withdrawal of the formation fluid 208. The controller 160 is connected to a system volume control device, such as pump 226. The pump 226 may be a progressive cavity pump or any suitable pump that can pump out formation fluid 208 from the formation 218. A flow meter is included to determine the fluid flow rate. A valve 230 for controlling fluid flow to the pump 226 is disposed in the fluid line 222 between the optical sensing module 170 and the pump 226. A test volume 205 is the volume below the retracting piston of the pump 226 and includes the fluid line 222.
The optical sensing module 170 is used to determine the refractive index of the formation fluid within the test volume 205. Any suitable optical module or system for determining the refractive index may be used. In one aspect, the optical sensing module is configured to measure the refractive index using reflection-intensity at a window-fluid interface. The refractive index is computed by comparing the reflection intensity of an air filled cell to the reduced reflection intensity when some other fluid is in the cell and using the known refractive indices of the window (typically a sapphire window) and of air. The refractometer of the optical sensing module 170, in one aspect, may be configured to measure refractive index in-situ of any desired fluid, including that of oil, gas and brine. The refractometer may have a broad range, such as from n=1.0 to n=1.75, and a relatively high resolution, such as 0.00025 or better. Such a refractometer provides a relatively broad refractive index range and has a relatively high resolution. Such a refractometer provides refractive index measurements that are useful for monitoring sample cleanup from mostly mud filtrate to mostly pure or connate formation fluid described herein. Any suitable refractometer may be used for the purpose of this disclosure, including but not limited to those described in U.S. Pat. No. 6,683,681 B2 and U.S. Published Application 2004/0007665 A1, each of which is assigned to the assignee of this application, and each of which is incorporated herein by reference.
The optical sensing module 170 is connected to the controller 160 to provide the feedback data for a closed-loop control system. The feedback is used to adjust parameter settings such as detecting sample clean-up. Sample clean up refers to the transition from filtrate-contaminated formation fluid to connate or nearly pure formation fluid while pumping fluid at selected depths in the wellbore. The downhole controller 160 may incorporate a processor, such as a microprocessor, for processing the reflective index measurements. A storage device, such as a memory device, may be used as a computer-readable medium to store data, computer programs and algorithms relating to the use by the apparatus described herein and to perform the various functions and methods relating to such apparatus.
During the clean-up process, the withdrawn fluid is vented to the upper annulus 130 via line 219. A conduit 227 connecting the pump 226 to the line 219 includes a selectable internal valve 232. If fluid sampling is desired, the fluid may be diverted to optional sample reservoirs or tanks 228 by using the internal valves 232, 233a, and 233b rather than venting the fluid through the line 219. The fluid contained in the reservoirs 228 is retrieved from the well for analysis.
In one aspect, the results of the data processed downhole may be sent to the surface for use and for further processing. The controller 160 passes the processed data to a two-way data communication system 116 disposed downhole. The communication system 116 transmits a data signal to a surface controller 150, which controller contains a processor 151 and memory storage device that stores computer programs, algorithms and data for use in the apparatus and methods described herein. Any suitable data communication system may be used for the purpose of this disclosure. The signals received at the surface are processed by the processor 151 associated with the surface controller 150, which converts and transfers the data to a suitable output and/or storage device 152. The surface controller 150 may also be used to send the test initiation commands to the downhole tool 103.
Still referring to
In one aspect, when collecting a water sample in a well drilled with water-based mud, well logs may be used to estimate the connate brine's resistivity and, from that, the connate brine's refractive index. As an example, the brine resistivity nay be estimated by using typical rock properties for the region of the wellbore, such as Archie parameters “a” and “m” of the resistivity factor (F=a/Porosity ^m) along with the deep-reading resistivity logs and neutron porosity logs over the water zone. Similarly, neutron logs may be used to estimate brine salinity. For a zone that is 100% saturated with water, the log-measured neutron cross-section equals Sigma_Log=Sigma_Brine*Porosity+Sigma_Matrix*(1−Porosity). From the effect of the dissolved salt on the brine's cross-section, brine salinity may be obtained by solving for Sigma_Brine in terms of typical neutron cross-sections for the formation of interest and the porosity as measured by neutron logs. Then from the brine resistivity and the pressure and temperature, brine's refractive index may be computed. The relationship between resistivity, pressure, temperature, and refractive index are discussed in U.S. Pat. No. 7,027,928. By either directly measuring the water-based mud filtrate's refractive index or by knowing its resistivity and then computing its refractive index, both end points from which to compute the percentage of contamination become known. That is, the refractive index of both the pure water-based mud filtrate (one endpoint) and the pure formation brine (the other endpoint) become known. The fraction of contamination may be computed as a linear interpolation between the two pure fluid endpoints.
When collecting an oil sample for a well drilled with oil-based mud, prior knowledge of produced fluid in the region to estimate the formation crude oil's refractive index may be used. If the oil based mud filtrate's refractive index is directly measured, then both end points are known from which the fraction of contamination may be obtained from a linear interpolation between the pure fluid endpoints.
Still referring to
In another aspect, the system of the present invention provides an estimate of the terminal purity or the contamination level by fitting a suitable curve to the refractive index data taken over a selected time period. For the purpose of this invention, any suitable curve fitting technique or algorithm may be used. Certain examples of curve fitting techniques that may be used for this invention are described later. A curve that may be fitted to the data of
In the example of
Resistivity of the formation fluid is related to the refractive index of the fluid and thus can be calculated from the refractive index and used to estimate the terminal values.
The curve fitting may be done by a processor in the downhole tool or at the surface processor or a suitable combination thereof. In one aspect, the measured data, such as shown in
In one embodiment, the method and apparatus of the present invention fit the measurement data to a non-asymptotic curve. One example of a non-asymptotic curve is a curve that provides a fit to the data over a typical or selected pumping time, such as between 30 minutes to two hours, and then extrapolates the results to several times the pumping time, but which approaches plus or minus infinity at infinite times, such as a power series approximation.
In one aspect, present invention fits a continuously-differentiable, non-asymptotic curve to the raw data. The fit can be to the elapsed time or to the volume of the fluid pumped. The present invention may use, for example, but is not limited to, fitting to the raw data points a non-asymptotic curve such as A(t)=c1+c2t1/2+c3t1/3+c4t1/4. Using calculus, the program analytically calculates the first derivative as dA/dt=(c2/2)t−1/2+(c3/3) t−2/3+(c4/4)t−3/4. For the purpose of using this method, A0 is denoted to be the “terminal” refractive index, i.e. the refractive index at some very long time (e.g., 24 hours), which time is much longer than the time at which the pumping is normally terminated. As time progresses, both (A0−A) and t(dA/dt) decrease, where A is the refractive index at time t. Assuming that both such values decrease at the same rate, then they are proportional to each other, which means (A0−A)=mt(dA/dt), where “m” is a constant. In one aspect, the present method tries various estimates for A0 until it finds an estimated value of A0 that produces an acceptable or the best linear, least-squares fit between y=(A−A0) and x=[t(dA/dt)]. The best fit is given by y=mx+b, where the intercept b is closest to zero, which has been determined to be more sensitive than finding the maximum “R2” for linear fits between two variables that are directly proportional. To perform the curve fitting, the present invention selects a raw data point at a selected time, t, (which may be the latest time, t) at which the actual data intersects (or gets closest to) the best fit line. To forecast refractive index at a slightly later time, t+Δt, the method uses ΔA=(A0−A)/[1+m(1+t/Δt)] which is obtained by replacing dA/dt by ΔA/Δt, replacing t by t+Δt, and replacing A by A+ΔA in (A0−A)=mt(dA/dt). The method then recursively applies this ΔA formula to forecast the refractive index at t+Δt and then uses the newly-calculated refractive index to compute the refractive index at a slightly later time, t+2Δt, and so on, for all future times. In this manner the method generates future forecasts for A(t).
In this method, if the slope m of the fit is positive, it indicates that an undesirable section of raw data has been selected, which is curving upward or downward towards plus or minus infinity. In such a case the method selects a new raw data point at some time, t, and continues the process of curve fitting, as described above. For data that is rising and leveling off over time, the fraction of terminal purity at any future time, t, is given by A(t)/A0. For data that is falling and leveling off over time, the fraction of terminal purity at any future time, t, is given by [AS−A(t)]/[AS−A0], where AS is the starting refractive index at the left edge (the earliest time) of the selected data window.
Thus, in one aspect, the present disclosure provides a method for estimating a characteristic or parameter of interest, such as a current, future or terminal purity or contamination of brine in the formation fluid being withdrawn or pumped at a selected location in the wellbore during the withdrawal of the fluid. In one aspect, the method may is for estimating a characteristic of brine in a fluid obtained from a formation that has water-based mud invasion. The method may include estimating a refractive index of a connate brine from well log measurements; withdrawing the fluid from the formation; measuring a refractive index of the fluid a plurality of times during the withdrawal of the fluid from the formation; and comparing the estimated refractive index with a refractive index measured during the withdrawal of the formation fluid to estimate the characteristic of the brine. The characteristic or property of interest may be the contamination level in the fluid sample or the purity of the brine in the fluid sample or fraction of pure or connate brine or that of contamination in the fluid sample.
In another aspect, the method may estimate a characteristic of brine in a fluid obtained from a formation that has water-based mud invasion, wherein the method includes: estimating refractive index of connate brine from well log data, such as resistivity or other electrical logs; withdrawing the fluid from the formation; measuring refractive index of the fluid a plurality of times during the withdrawal of the fluid; fitting a curve to data values that correspond to the plurality of refractive index measurements; and estimating the characteristic of the brine from the estimated refractive index and the fitted curve. The method may further include estimating a terminal value of the data values and/or estimating a contamination level or purity level of the brine at a future time from the estimated refractive index and the fitted curve. The data values to which the curve is fitted may be the actual refractive index measured values or resistivity values that are derived from the measured refractive index values. In one aspect, the withdrawing of the fluid is terminated when the difference between the estimated refractive index and the terminal value is greater than a selected value. In another aspect, a formation fluid sample is collected when the purity of the brine is determined to be of an acceptable level. In another aspect, the estimated refractive index of connate brine is computed corresponding a selected temperature and pressure.
In another aspect, a method is provided for estimating a characteristic of a fluid during the withdrawal of the fluid from a formation, which method includes: withdrawing the fluid from a formation; measuring a refractive index of the fluid during the withdrawal of the formation fluid to obtain a plurality of refractive index values; obtaining a plurality of resistivity values corresponding to the plurality of refractive index values; fitting a curve through the plurality of resistivity values; estimating a terminal value of the resistivity values from the fitted curve; and estimating the characteristic of the fluid using a current resistivity value and the estimated terminal value. The characteristic of the fluid may be a terminal value of contamination in the fluid; terminal value of purity of the fluid; terminal value of a hydrocarbon content in the fluid; fraction of contamination in the fluid; fraction of oil in the fluid; fraction of gas in the fluid; or fraction of water in the fluid. Any of the methods may estimate components of the fluid and display a visual image of the estimated components that may be a gray scale visual image or a color visual image and each such image may be a two-dimensional or a three-dimensional image.
In another aspect, apparatus for estimating a characteristic of brine in a fluid withdrawn from a formation is disclosed that may include a: probe to withdraw the fluid from the formation; a refractometer that provides refractive index measurements of the fluid during the withdrawal of the fluid from the formation; a storage device that stores an estimated value of a refractive index of connate brine in the formation that is obtained using well log data; and a processor that estimates the characteristic of the brine from the estimated value of the refractive index of the connate brine and refractive index measurements made during the withdrawal of the fluid from the formation. A sample chamber may be used to collect a fluid sample. A pump may be used to pump the fluid from the formation into the sample chamber against hydrostatic pressure. Compressed gas in a chamber may be used to pressurize the fluid in the sample chamber. In another aspect, the processor may fit a curve to data values that correspond to the plurality of refractive index measurements and estimate the characteristic of the brine in the withdrawn fluid from the estimated refractive index of the connate brine and the fitted curve. The processor also may estimate a terminal value of the data values and estimate a contamination level or purity level of the brine in the withdrawn fluid at a future time from the estimated refractive index of the connate brine and the fitted curve. The data values used for fitting the curve may be the data values that correspond to measured values of the plurality of the refractive index measurements or resistivity values corresponding to the plurality of refractive index measurements. In another aspect, the processor may be configured to cause the taking of a sample of the fluid from the formation when a selected data value indicates that the purity level of brine or the contamination level in the brine is acceptable.
In another aspect, a computer-readable medium is provided that has embedded therein a computer program, which may include: a set of instructions to fit a curve to data corresponding a plurality of refractive index measurements of a fluid taken during withdrawal of the fluid from a formation; a set of instructions to estimate a terminal value of the refractive index from the fitted curve; and a set of instructions to estimate a characteristic of brine in the fluid from the fitted curve and an estimated value of connate brine computed from using well log data. The computer program may further include a set of instructions to fit the curve to the data values over a selected time and extrapolate the fitted curve to a multiple of the selected time that approaches plus or minus infinity at infinite time.
The computer-readable medium may be a ROM, RAM, CD ROM, DVD, FLASH or any other computer-readable medium, now known or unknown, that, when executed, causes a computer such as, for example, a processor in downhole controller 418 and/or a processor in surface controller 412, to implement the methods of the present invention.
The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible. It is intended that all such changes and modifications be interpreted as part of the disclosure.
DiFoggio, Rocco, Simpson, Angus
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